PROJECT SUMMARY Metastatic hormone-sensitive prostate cancer is treated with androgen deprivation therapy and after progression to the castration-resistant prostate cancer stage, with additional forms of hormonal ablation and/or with chemotherapy. These treatments slow tumor progression by a certain period and decrease the pain and suffering from disease progression. However, there is no clinical feature or molecular test that can reliably predict these treatment responses or clinical outcomes in advanced prostate cancer. A predictive feature or biomarker which is defined as a factor determining which patients will do well with a specific type of treatment. This is distinct from molecular prognostic biomarkers which provide information about clinical outcome regardless of therapy used. Knowledge of predictive factors that identify cohorts of patients destined for response (versus failure) of these therapies is critically needed to develop precision medicine strategies. Recently, the assessment of tumor-derived cell free nucleic acids in body fluids has shown promise in being able to capture tumor genomic and genetic abnormalities in cancer patients. This approach is often referred to as ?liquid biopsy?. We believe that cell free nucleic acids can be used as biomarkers to reliably predict treatment response and clinical outcomes, and will therefore be informative in designing an appropriate treatment regimen. We have previously examined plasma cell free nucleic acids for miRNA abundance and somatic DNA changes in patients with advanced prostate cancer. We observed a significant association of high plasma miRNAs (miR-375 and miR- 1290) with poor overall survival. We also observed that tumor-derived genomic/genetic alterations in plasma cell free DNAs were associated with tumor burden and reflected patients' responses to stage-specific treatments. Aim 1 of this study is to identify and validate key circulating miRNA-based predictive and prognostic biomarkers in four well annotated prostate cancer cohorts. Aim 2 is to establish circulating cell free DNA-based predictive and prognostic biomarkers in four well annotated prostate cancer cohorts. Aim 3 is to functionally characterize the potential of the key candidate miRNAs in promoting growth and resistance of prostate cancer cells to androgen deprivation therapy or chemotherapy in vitro and in vivo. The proposed studies are highly significant and timely, as the circulating cell free nucleic acid-based biomarkers will not only help clinicians in selecting the most effective treatment options, but also provide important clues regarding mechanisms that underlie prostate cancer progression and recurrence.